In radiation therapy, a target volume, a tumor, for example, in a patient is irradiated with ionizing radiation. Here, external radiation therapy, which includes irradiation of a patient's body from outside the body, is known. Internal radiation therapy, also referred to as brachytherapy, is also known. In brachytherapy, radiation sources that include radioactive substances are placed inside a patient's body in order to damage or destroy the tumor tissue locally in the patient's body in the target volume.
Basically, when irradiating the patient, it is a challenge to ensure that an adequate dose of radiation is applied to a target volume such that the tumor tissue contained in the target volume is destroyed. At the same time, organs at risk surrounding the target volume need to be spared as much as possible. Hence a high level of accuracy is very important in the planning of the radiation treatment.
Planning and/or monitoring radiation therapy or radiation treatment of a patient by way of imaging is/are known. A radiation treatment plan is usually drawn up for this purpose with the aid of medical imaging data from the patient that has been generated using a three-dimensional imaging method. Computed tomography image data (CT image data) is generally used for this. From the computed tomography image data, on the one hand the target volume for radiation therapy can be established, and on the other hand, surrounding organs at risk can be located.
Furthermore, the intensity values for the image voxels in the image data (measured in “Hounsfield Units”) depict a close approximation of an electron density at the corresponding location in the patient's body because the intensity values for the image voxels are based on an absorption of X-rays at the relevant locations. In this way, the computed tomography image data for planning the radiation treatment can be converted into an electron density map in a really simple way.
In a radiation treatment, because the intensity of the interaction of the radiation is correlated with the electron density in the body, the attenuation of the radiation as it goes through the body can be calculated in a comparatively simple manner from the computed tomography image data. Due to this property, computed tomography image data has hitherto been used preferentially when drawing up a radiation treatment plan. Furthermore, computed tomography image data has only a slight geometrical distortion and consequently allows an appropriate definition of a reference geometry for the planning of the radiation treatment and carrying out the radiation treatment.
Nevertheless, there is a demand for other imaging methods that have a better soft tissue contrast to be used when planning the radiation treatment in order to allow an improved identification of the tumor tissue in the target volume and/or of the organs at risk. Such an imaging method that meets the demand for a better soft tissue contrast is magnetic resonance imaging (MR imaging) using a magnetic resonance unit. In such imaging, the contrast is dependent on the distribution of the spin density, the interaction of the spins with one another and/or with their surroundings. In this way, a soft tissue contrast can be achieved that is clearly superior to the contrast achievable with a computer tomography system.
In a magnetic resonance unit, also known as a magnetic resonance tomography system, the body that is to be examined of an examination subject, in particular of a patient, is exposed with the aid of a main magnet to a relatively high main magnetic field, of for example 1.5 or 3 or 7 Tesla. In addition, gradient pulses are applied with the aid of a gradient coil unit. High frequency pulses, in particular excitation pulses, are then emitted via a high frequency antenna unit using appropriate antenna devices, which leads to the nuclear spins of certain atoms, resonantly excited by these high frequency pulses, being flipped round a defined flip angle against the magnetic field lines of the main magnetic field. When the nuclear spins are relaxed, high frequency signals known as magnetic resonance signals are emitted, which are then received by appropriate high frequency antennas and then further processed. The desired image data can then finally be reconstructed from the raw data acquired in this way.
Such a combined use of computer tomography imaging and magnetic resonance imaging is known for planning a radiation treatment. For the planning of the radiation treatment, the acquired computed tomography image data and magnetic resonance image data are then typically superimposed by way of image registration. In so doing, however, the problem may occur that an image registration that is not carried out completely correctly can introduce systematic errors into the planning of the radiation treatment. In addition, with the combined use of computer tomography imaging and magnetic resonance imaging, the image data has to be acquired with the two modalities spaced a short distance away from each other so that changes in the patient's anatomy, due for example, to differences in how full the patient's bladder is, can be avoided from one data acquisition to another. This can lead to operational challenges, increased costs, and reduced patient comfort as the patient has to suffer a number of examinations.